44 THE TAPE-WOBM. 



ing to its genus and species, receives a specific, ramified, folia- 

 ceous, or encrusting form, whilst in the compound tape-worms, 

 the individuals only grow out of the common stock in one 

 direction, and in a single series. 



In the Cestoidea the stock is the posterior end of the scoliciform 

 agamozooid. In the alternation of generations amongst the 

 Cestoidea, there is this peculiarity, that the agamozooid preserves 

 its efficacy and independence, whilst the agamozooids of other 

 animals which undergo alternation either die after producing 

 their brood or pass into it. 1 



We must consider the head of every cestoid worm as the 

 agamozooid still remaining and capable of reproduction, and its 

 neck as the equivalent of the posterior extremity of the scolex. 

 In all cestoids we see that fresh joints are continually being 

 developed at the posterior part of the neck, which lengthens 

 and becomes covered with transverse folds. These folds are at 

 first very close together, but as the process of growth throws 

 them backwards, further and further from their place of origin, 

 they gradually change from indistinct wrinkles into sharp trans- 

 verse lines of demarcation, between which the substance of the 

 body dilates into a joint (individual), and assumes its specific 

 shape. At a later period, the rudiments of the hermaphrodite 

 sexual apparatus make their appearance in the interior of the 

 joints : and in proportion as the latter move backwards from their 

 parent stock (the neck), so much the nearer do they approach 

 to maturity, through progressive development of their sexual ap- 

 paratus ; and finally they separate themselves from their younger 

 fellows as independent individuals. I must not leave the fact 

 unmentioned that the formation of marked proglottidiform joints 

 does not take place in all Cestoidea. In the genera T&nia, 

 Tetrarhynchus and several others furnished with cephalic hooks 

 and suckers, the development and individualising of the pro- 

 glottides occurs in perfection. In the genus Bothriocephalus , 

 although the joints exhibit a distinct demarcation, they show 

 little inclination to become separate. In Triaenophorus the 

 articulation is still less marked; whilst in Ligula it is obso- 



1 The correctness of this statement appears to be doubtful. The stock of the 

 "Hydra tuba' 1 remains after giving rise to a brood of Medusae, and neither dies nor 

 can be said to pass into the brood. The like is true of those Sertularida, Diphyda, and 

 PhysophoridcB which give rise to medusiform Zobids ; nor does it seem to be otherwise in 

 the remarkable Trematode Gyrodactylus described by Von Siebold himself. [D.] 



